1. Field of the Invention
The present invention relates to medical instruments generally and more particularly to an auditory-evoked-action potential-measuring system for use in measuring hearing acuity, the system having means for automatically testing the system and for selecting the larger of two auditory-evoked-action potentials.
2. Description of the Prior Art
It is generally accepted that the acquisition of basic vocabulary and language skills, including the acquisition of the basic units (words and speech sounds) and the rules for their combination (grammar and syntax), occurs during the period when a child is from six months to two and a half or three years of age. Obviously, the acquisition of such skills, and skills which depend thereon, may be significantly retarded if a child has a substantial undiagnosed hearing defect. Although spectacular diagnoses have been made by highly-skilled operators (audiologists) employing behavioral-type instruments such as continuous-tone audiometers to generate tones of selected frequency and level and observing the child's (subject's) response thereto, conventional hearing acuity measurements with such instruments rely upon a cooperating subject to report threshold levels. Obviously, such cooperation (tasks) are beyond the skill levels of children this young.
A prior-art-type system which does not require such cooperation directly measures the evoked-action potentials developed by a subject responsive to a stimulus. To facilitate the separation of the evoked-action potentials from other (EEG-type) brain-activity potentials, the above-mentioned prior-art-type evoked-action-potential measuring system employs a generator, called a pip generator, for periodically developing a signal from which a periodic acoustic stimulus may be developed. The generator periodically develops a tone-pip signal the frequency of the cycles of which occur at an auditory rate and the amplitude of the cycles of which is defined by an envelope having an attack portion and a decay portion both chosen to avoid "clicks." Additionally, the prior-art-type evoked-action-potential-measuring system employs circuitry for amplifying and filtering the evoked-action potentials and a cathode-ray-type unit for displaying the filtered potentials so that a trained and experienced operator may ascertain the evoked-action-potential levels (and possibly other parameters such as the latency thereof).
Evoked-action-potential-measuring systems are advantageous in that they permit the validation of auditory (and possibly other) systems of very young children, preferably as part of a standard well-baby check-up at 6 months of age. Unfortunately, the above-mentioned prior-art-type evoked-action-potential-measuring system is relatively complex and expensive and requires for its use the services of a trained and experienced operator.
Another problem is that evoked-action potentials are not always generated so as to appear symmetrical. In other words, it has been observed that with a significant number of subjects (perhaps 50%) the evoked-action potential measured at a lateral point (such as on the earlobe, on the mastoid or in the ear canal) on one side of the head with respect to a common point (such as the forehead) differs significantly in amplitude (perhaps by a factor of 2) from the amplitude of the potential measured on the other side. This lateralization is apparently independent of the side stimulated and does not appear to be related to dominance (as in right/left handedness). A low evoked-action-potential level may also be traced to an excessive skin-electrode-contact impedance.